Children of Parents with Mental Illness (COPMI) have an increased risk of maltreatment and neglect, behavioural, social and educational difficulties, and mental and physical health problems (Diggins, 2011; Stanley, 2003). The problems arise not only because parents with serious mental illness find it difficult to manage their role as children's carers, but also because they often live in deprived conditions and have the ongoing stressor of their children potentially or actually being removed from home (Park, Solomon, Mandell, 2006).Evidence-based interventions to improve these children's quality of life are scarce (Bee et al, 2014). Further intervention development and evaluation should focus on improving the children's coping mechanisms and mental health literacy and should use measures that capture their specific needs (Bee et al, 2013). Family SMILES (Simplifying Mental Illness + Life Enhancement Skills) is a 3-month family-centred intervention delivered in few parts of the UK by the National Society for the Prevention of Cruelty to Children (NSPCC) to 8-13 year-olds who have mentally ill parents. Family SMILES comprises children's group sessions, parents' one-to-one sessions and joint parent-child sessions, with the aim of enhancing the children's resilience and self-esteem, reinforcing their parents' protective function, and improving parent-child communication and family relationships. Our 3-year research project will adapt the NSPCC's Family SMILES programme to produce a new manualised intervention, called Young SMILES, which focuses on improving the children's health-related quality of life (HRQoL). We will evaluate the feasibility, acceptability and outcomes of Young SMILES for children and families who see or receive support from the NHS. Families whose children aged 6-16 have at least 10 hrs/week contact with a seriously mentally ill parent/carer will be invited to participate. The intervention will include children at risk of maltreatment or neglect and those with childhood mental health problems as well as children without any current or emerging health or social problems. Phase 1 (months 1-9) will draw on focus groups and individual interviews with stakeholders (children and parents/carers with severe mental illness, adults who grew up with mentally ill parents, professionals from health and social care, education, and the voluntary sector) to co-produce Young SMILES and determine optimal age-clustering for the children's groups as well as optimal duration, interval, format, content and delivery mode for both the children's and parents' sessions. Phase 2 (months 10-30) is a feasibility Randomised Controlled Trial (RCT), which will determine rates of recruitment, randomisation, intervention adherence and retention to follow up, describe the outcomes and costs of the intervention added to usual care against usual care alone, and indicate optimal outcome measures and costing methodologies for a future definitive RCT. A maximum of 60 families will be recruited over 15 months across the North East of England, with two hubs in Middlesbrough and Newcastle. Practitioners from the NHS, Social Care and the Voluntary Sector will deliver the intervention in a combination of community settings and family homes. We shall collect data at baseline (randomisation) and at months 3 (primary endpoint), 6 and 12 post-randomisation, on children's HRQoL, mental health literacy and problem-solving, depression and anxiety, family functioning and parent-child relationship, health and social care utilisation. We shall use two standardised child-rated questionnaires to measure child HRQoL: the PEDQOL (Calaminus et al, 2000) and the KIDSCREEN (Ravens-Sieberer et al, 2007). Both measures have been validated for our population's age group (6-16 yrs) and show sensitivity to change in quality of life domains such as physical/ mental/ emotional health and social/school functioning. Outcome measures will include the Strengths and Difficulties Questionnaire (SDQ, Goodman et al, 2000), which is routinely used by IAPT-CYP and NSPCC services, and the Revised Child Anxiety and Depression Scale (RCADS, Chorpita et al, 2005). We shall use relevant case vignettes with follow-up questions similar to Jorm et al (1997) to assess the children's mental health literacy and problem-solving skills. We shall measure parenting skills and child-parent relationships using the Arnold-O'Leary parenting scale (Arnold et al, 1993) and the Parenting Stress Index Short Form (Abidin, 1979). For the economic analysis, we shall use the CHU-9D (Stevens, 2012), which has been validated for children aged 7-17, to estimate incremental health gain in quality-adjusted life years (QALYs). Resource use will be assessed using a modified version of the Adult Service Use Schedule (Ad-SUS) (Byford et al, 1999) which will be revised and piloted for use with our participating families. Phase 3 (months 31-36) will be for data analysis and reporting of the study's findings. We shall also prepare a protocol for a definitive RCT, should our findings warrant it. Our team of co-applicants and collaborators includes adult and young service users as well as professionals with expertise in child and adult psychiatry, third sector organisations, social work and family law, nursing, psychological therapies, NHS service delivery and management, statistics, qualitative methods, health economics and user-led research. We shall work with the NSPCC as part of our dissemination plan to draw up an open IP licence agreement which will enable the large-scale use of Young SMILES on a not-for-profit basis. We shall work with national and local service user groups and agencies, such as Rethink Mental Illness and Young Carers, to communicate and discuss our findings.